1. Field of the Invention
The present invention relates to a low-loss acoustic surface wave transducer with improved spurious reflection components.
2. Description of the Art
As a surface acoustic wave transducer, a type with a construction in which a first and a second comb-shaped electrodes are interdigitated with a certain gap on the surface of a piezoelectric substrate (I.D.T.) has been practically used. In such a device it is possible to realize a variety of frequency characterstics by varying the pitch and the interdigitated length of the electrode strips of an IDT.
As spurious components which invariably are troublesome in various surface acoustic wave devices constructed by utilizing such an IDT, there are spurious reflection components of the surface acoustic wave due to the IDT. When a filter or a delay line is actually constructed by using such an IDT, these spurious reflection components appear in the form of the triple transit echo (TTE), acting as a cause for deteriorating the characteristic of such a device markedly.
The factors of the spurious reflection components are the reflection components due to the difference in the acoustic impedances of the electrode portion and the gap portion of the IDT (referred to as R.sub.A hereafter) and the reflection components due to the electric re-excitation of the IDT(Referred to as R.sub.E hereafter).
Therefore, it will be seen that in order to remove the spurious reflection components it is necessary only to cancel the reflection components R.sub.A and R.sub.E by giving them opposite phases.
In the past, there has been disclosed in Japanese Patent Publication No. 58-84517 a transducer which is arranged to have the reflection components R.sub.A due to the acoustic impedance difference and the reflection components R.sub.E due to the electric re-excitation cancel each other based on the above technique. The transducer includes a piezo-electric substrate on which a first comb-shaped electrode and a second comb-shaped electrode are formed. The electrode-strips sets of the transducer comprise a first type of electrode-strips set A which generates the electric reflection R.sub.E but no acoustic reflection R.sub.A, and a second type electrode-strips set B which generates both of the electric reflection R.sub.E and the acoustic reflection R.sub.A. The strip widths of the first type of electrode-strips set A are set at .lambda./8 and 5 .lambda./8, while the strip widths of the second type electrode-strips set B are set as a combination of .lambda./8 and 3.lambda./8, where .lambda. is the wavelength of the surface acoustic wave.
The region covered by the electrode type B forms the shape of a triangle. That is, since the electric reflection is generated at both regions A and B, the wave form on the time axis of the resultant electric reflection characteristic of the IDT becomes the shape of a triangle which represents the result of the self-convolution of a quadrangle. Therefore, to cancel out the electric reflection R.sub.E at all the points on the time axis it is sufficient to choose the wave form of the acoustic reflection R.sub.A as a triangle with the shape as the triangular region of the electrode type B. Translated in terms of an event on the frequency axis, it is required that the frequency-amplitude characterstic of the acoustic reflection Ra and the frequency-amplitude characteristic of the electric reflection R.sub.E be arranged identically over a wide range of frequency. In other words, when a filter is constructed by an IDT with the above range of frequency, it should be possible to restrain TTE over a wide range of frequency. However, when the overall reflection characteristic is examined by an actually constructed IDT with the above structure, it was observed that the overall reflection charcteristic did not disappear on the entire range of the time axis but left spurious components over small time ranges corresponding to the vicinity of the extremities of the IDT.